The NMR technique is based upon the magnetic properties of nuclei containing odd numbers of protons and neutrons. These nuclei possess an angular momentum related to the charge thereof. The magnetic moment is directed along the spin axis of each nucleus. When placed in a strong and generally homogeneous static magnetic field, designated B.sub.o, the nuclei either align with or against the applied field and precess with a common sense about the applied field. The precessional angle of a nucleus may be changed by absorption of electromagnetic energy through a phenomenon known as nuclear magnetic resonance, NMR, which involves impressing upon the nuclei a second rotating magnetic field, designated B.sub.1, of frequency to match that of their normal precession. When the applied RF magnetic field is removed, the nuclei precess and relax toward their equilibrium conditions, generating RF signals characteristic of the molecular environments in which the nuclei reside. The frequency at which they precess is known as the Larmor frequency and is given in annular frequency by .omega.=.gamma.B. .gamma., the gyromagnetic ratio, is a constant for each nucleus or nuclear isotope and generally results in widely separated Larmor frequencies for a given applied field strength, B.sub.0. B is the magnetic field acting on the nuclei and is modified by the molecular environment of a nucleus according to B=B.sub.0 (1-.delta.). .delta. is the chemical shift offset impressed upon chemically equivalent nuclei by the local electronic distribution. Measured usually in parts per million, chemical shifts of a particular nucleus or nuclear isotope produce much smaller differences in frequency, and spectra derived from them can be used to obtain quantitative, structural, and dynamic information about the molecules of a sample. Because the Larmor frequency is proportional to the applied field B.sub.0, the resonance frequencies of chemically equivalent nuclei will vary across the sample according to the strength of the magnetic field. It is only with technical difficulty that homogeneous B.sub.0 fields are obtained, and high-field magnets are usually provided with electronic shim coils to counter both residual distortions of the magnet and the susceptibility distortions from sample or tissue being investigated or from materials comprising NMR probe. Acquisition of highly resolved spectra from a sample is usually preceded by a "shimming" procedure using a high sensitivity NMR signal from hydrogen protons or another abundant nucleus.
In performing medical NMR spectroscopy, the NMR instrument is generally configured to observe a single nucleus such as hydrogen protons (1H), phosphorus-31 (31P), or carbon-13 (13C). Since phosphorus containing metabolites are key indicators of the state of tissue, considerable effort has been directed towards acquiring and analyzing phosphorus spectra from tissue. Acquisition of high sensitivity phosphorus and other spectra from human tissue has been utilized for identifying and characterizing tissues and following their response to treatment. In another configuration, a bias or gradient in the normally homogeneous B.sub.0 field is introduced across the sample for the purpose of spatially encoding information into the NMR signals. Images are later reconstructed from the information contained within this data, forming the basis of NMR imaging, a technique now widely used in medical diagnostics. The homogeneity of the B.sub.0 field is reflected in the quality of its proton images, that is more homogeneous fields produce images with less distortion intensity.
The B.sub.1 field for transmitting to the sample is derived most efficiently from a resonant RF coil placed in proximity to the sample and connected to the RF transmitting apparatus. Either the same or a second RF coil may be connected to the RF receiving apparatus to receive the NMR signals, which are induced in the coil by the precessing magnetism of the nuclei. Free induction signals from chemically shifted nuclei and from samples with B.sub.0 field gradients impressed upon them are normally received with a single-resonant coil tuned to the Larmor frequency of the nucleus. The B.sub.1 field generated by this receiving coil must be homogeneous over the sample to produce more uniform spectral measurements and images.
It well known that improved sensitivity and a reduction in transmitter power can be obtained if a coil can be operated in circularly polarized mode. See C.-N Chen, D.I. Hoult, and V.J. Sank, J. Magn. Reson 54, 324-327 (1983). A linear oscillating field, such as produced by a simple resonant coil, can be cast as the sum of two circularly polarized components of equal amplitude. Likewise, by combining out of phase the linearly oscillating fields of two well-isolated, single-tuned crossed coils or the well-isolated fundamental modes of a multi-modal structure such as the "birdcage" coil, described later, a single, circularly polarized magnetic field can be produced which matches the precessional motion of the nuclei. Circularly polarized coils are similar to crossed-coil double-tuned probes in that two resonant circuits require tuning. They differ, however, in that being of the same frequency, they require a high degree of electrical isolation to operate independently, as will be shown later.
With the conventional "birdcage" coil, improved homogeneity in planes perpendicular to the coil axis is achieved with currents distributed sinusoidally in the straight conductors of the coil. The finite length of the straight conductors and the currents flowing in the end rings contribute to an inhomogeneous field in the interior of the coil. Improved homogeneity parallel to the coil axis is obtained by increasing the coil length thereby increasing the length of the straight conductors and moving the end rings away from the coil center. A trade-off exists, however, since lengthening the coil reduces the coil sensitivity. The resonator of the present invention redistributes the currents in the coil, concentrating them in two bands of conductors at either end. B.sub.1 homogeneity is thereby improved along and in the region of the longitudinal axis without increasing coil length. By maintaining the sinusoidal current distribution about the coil axis, homogeneity in planes perpendicular to the coil axis is maintained.
It is therefore an object of this invention to provide an RF resonator with currents concentrated in outer bands of the coil to provide a more homogeneous field profile along and in the region of the coil axis.
It is another object of this invention to provide an RF resonator with currents distributed sinusoidally about the coil axis to provide a substantially homogeneous B.sub.1 field in planes perpendicular to the coil axis.
It is another object of the invention to provide an RF resonator capable of circularly polarized operation with improved signal to noise ratio over volume of interest.
It is yet another object of this invention to provide an RF resonator with plurality of conductors employed to construct the coil and tuning capacitance distributed along the outer bands such that the entire coil resonates at any one given NMR frequency.